Slightly soluble inorganic salts, for example calcium carbonate, calcium sulfate, and barium sulfate, often precipitate from waters produced along with oil and gas. Such precipitation occurs because conditions that affect solubility, for example temperature and pressure, change, or because incompatible waters mix in, or near, producing well bores. Precipitates of slightly soluble salts are often called scale. Scale build up often occurs within the near wellbore formation and the wellbore equipment. Scale buildup within the formation decreases permeability and impairs well productivity. Scale build-up within wellbore equipment shortens equipment useful life and can plug production tubing.
To minimize scaling, scale inhibitors can be placed within the formation by "squeeze" methods. Squeezing involves injecting the inhibitor and, usually, overflushing the treated zone with water or brine. The well is then placed back into production. Inhibitor in the produced waters protects the wellbore and downhole equipment from scale build-up.
Squeezing inhibitor into the formation is typically a preferred way to apply an inhibitor. Squeeze injection eliminates any need for continuous injection, and the formation in the vicinity of the wellbore is protected. Most common scale inhibitors are effective in concentrations greater than about 1 to 100 parts per million. In a normal squeeze placement, it is difficult to control the concentration of inhibitor returning in produced brines. The inhibitors tend to be produced quickly, with concentrations quickly tailing off to ineffective amounts. This results in frequent shutdowns of production for squeezing operations, and also in excessive chemical costs.
Various attempts to control and delay inhibitor production have been only partially successful. For example, U.S. Pat. No. 3,483,925 discloses a method to provide a more gradual return of inhibitor by injection of the inhibitor with polymer-thickened slugs of solution. Injection of such thickened solutions would inhibit production of all fluids from the wellbore, and is therefore not a desirable method to provide a more gradual return of the inhibitor when production from the wellbore is desired.
U.S. Pat. Nos. 3,633,672 and 3,704,750 discloses inhibitors which are only slightly soluble in neutral or basic solutions and soluble in acidic solutions. These inhibitors are squeezed into the formation in an acidic solution, and then dilution with formation brine and reaction with reservoir rock within the formation raises the solution pH and causes precipitation of the inhibitor. The inhibitor is then produced gradually due to a slight solubility in formation brine. This method solves many of the problems with squeeze placement of scale inhibitors, but due to plug flow into the formation and then back with production, much of the inhibitor is not precipitated into the formation within a reasonable amount of time. Further, placement of the precipitate is poorly controlled and not predictable.
U.S. Pat. No. 4,357,248 discloses an improvement to processes such as those disclosed in U.S. Pat. Nos. 3,633,672 and 3,704,750. Patent '248 discloses injection of a scale inhibitor in a form which is soluble in a high pH aqueous solution and insoluble in neutral or low pH solutions. The solution further comprises a compound that hydrolyzes to form acidic compounds at a rate which causes the inhibitor to precipitate in the formation without the need for mixing with formation brines or reaction with formation rocks. This method, in spite of its advantages, has not been widely practiced because of difficulty in handling the useful pH lowering materials.
U.S. Pat. No. 4,860,829 discloses a method to precipitate calcium salts of phosphonate scale inhibitors in formations wherein the calcium is injected complexed with a chelate, along with a phosphonate scale inhibitor in the form of a sodium salt. The calcium is gradually released by the chelating agent and causes the phosphonate scale inhibitor to precipitate as a calcium salt. With this method of precipitating the scale inhibitor in the formation, precipitation begins once sufficient calcium is released from the chelating agent to saturate the solution with the calcium salt of the scale inhibitor. Equilibrium levels of the calcium salt of the scale inhibitor and chelate are readily determined, although the rate at which that equilibrium is reached is not easily determined and is dependent upon many variables.
U.S. Pat. Nos. 5,141,655 and 5,211,237 disclose scale inhibitor compositions and methods to place scale inhibitors in subterranean formation. The scale inhibitors are injected in an acidic solution, and the pH of the acidic solution is raised within the formation by a heat sensitive pH raising substance. The heat sensitive pH raising substance raises the pH of the inhibitor solution within the formation. The solubility of the scale inhibitor at the higher pH is significantly reduced. The scale inhibitor therefore precipitates within the formation as a calcium salt when the pH of the composition increases. Amides and ureas are known heat-sensitive pH raising components. A shortcoming of this method is that the reaction wherein high pH components are produced from the precursors is sensitive to the presence of catalysts such as iron ions. Iron ions are present in abundance in most wellbore and production tubing environments. These iron ions accelerate the hydrolysis of amides and ureas to such an extent that the pH of the inhibitor solution is raised, causing precipitation of the scale inhibitor before the inhibitor is placed in the formation.
A need therefore remains in the art for a scale inhibitor injection method wherein the scale inhibitor can be squeeze injected into a formation as a solution and then precipitated within the formation.
It is therefore an object of the present invention to provide a method to inhibit scale formation using a squeeze method wherein the inhibitor is injected as an aqueous solution from which the inhibitor precipitates. It is a further object to provide such a method which does not require the use of chemicals which are difficult to safely handle. It is yet a further object to provide such a method wherein initial precipitation of the scale inhibitor is delayed until after a time period sufficient for squeeze injection into a subterranean formation.